1. Field of the Invention
The present invention relates to a paper feed scanner, and more specifically, to a method of using a paper feed scanner to capture correct document images and ignore black areas surrounding the document images.
2. Description of the Prior Art
Paper feed scanners are popular for scanning large quantities of documents at one time. One main advantage that paper feed scanners have over flatbed scanners is that a stack of paper can be scanned all at once with little effort expended by a user.
In this specification, paper feed scanners include scanners which has automatic feeders for feeding documents page by page. Therefore flatbed scanners with automatic feeders are included in the “paper feed scanners” as described in the specification.
Please refer to FIG. 1. FIG. 1 is a paper feed scanner 10 according to the prior art. The paper feed scanner 10 comprises a housing 12, a paper tray 14, a step motor 16, a roller 18, a processor 26 and a scanning module 20. The paper tray 14 is connected to the housing 12 for holding a stack of documents 17. The step motor 16 is installed inside the housing 12 for driving the roller 18 so as to feed the documents 15 into the scanner 10. The processor 26 is used to control operations of the scanner 10. The scanning module 20 is installed inside the scanner 10 for scanning the documents 15 so as to generate document images. The scanner 10 has a scanning position positioned above the scanning module 20. When a document 15, one of the document stack 17, driven by the roller 18 reaches the scanning position, the scanning module 20 will start to scan the document 15, and the generated document image will be stored into a memory 22 of the scanner 10.
The scanner 10 further comprises a light sensor 24 for detecting the document 15. The distance between the light sensor 24 and the scanning position is a fixed value. The fixed value is converted to the corresponding number of steps rotated by the step motor 16 to deliver the document 15 from the light sensor 24 to the scanning position. This number of steps is recorded in the memory 22. When a front edge of the document 15 is detected by the light sensor 24, the step motor 16 will turn the predetermined number of steps before the scanning module 20 begins to scan. Similarly, when a rear edge of the document 15 is detected by the light sensor 24, the step motor 16 will turn the predetermined number of steps before the scanning module 20 stops to scan because the document 15 should have completely left the scanning position
Unfortunately, if the light sensor 24 is not accurately installed inside the scanner 10, an error will exist in the distance between the light sensor 24 and the scanning position. However, the number of steps recorded in the memory 22 is predetermined, it does not change with the error. Thus, documents 15 fed into the scanner 10 will not be properly scanned because the documents 15 may be moved too far or too little by the step motor 16 before the scanning module 20 begins scanning the documents 15.
For an illustration of this problem, please refer to FIG. 2 and FIG. 3. FIG. 2 illustrates a prior art scanning process in which the scanning module 20 starts scanning a document 15 too early. FIG. 3 illustrates a prior art scanning process in which the scanning module 20 starts scanning a document 15 too late.
In FIG. 2, the position error of the light sensor 24 lengthens the distance between the light sensor 24 and the scanning position. Before the front edge of the document 15 is dragged to the scanning position, the scanning module 20 begins scanning at line 28a. Because even though the document 15 has not reached the scanning position, the step motor 16 has rotated the predetermined number of steps. The scanning module 20 thus erroneously assumes the document 15 has reached the scanning position. Because the scanning module 20 starts to scan the document 15 too early, the scanning module 20 will scan a background section first, and then followed by a portion of the document 15. If the background color is black, then the scanning module 20 will acquire a document image containing a black area and part of the document 15. When the light sensor 24 detects a rear edge of the document 15, the scanning module 20 stops scanning the document 15 after the step motor 16 rotates the predetermined number of steps. This will cause the scanning module 20 stops scanning the document 15 at line 28b. That is, scanning is stopped before the rear edge of the document 15 is completely moved across the scanning position because the scanning module 20 erroneously determines the rear edge of the document 15 has reached the scanning position. Therefore, the acquired document image fails to contain a complete image of the document 15.
On the other hand, in FIG. 3, the position error of the light sensor 24 shortens the distance between the light sensor 24 and the scanning position. Therefore, after the front edge of the document 15 has already been dragged across the scanning position, the scanning module 20 begins scanning at line 30a. The document 15 is then moved completely across the scanning module 20, and the scanning module 20 stops scanning at line 30b. Therefore, a document image is acquired that contains part of the document 15 and a black area. And a very beginning portion of the document 15 was not scanned.
When installing the light sensor 24 into the scanner 10, the position error of the light sensor 24 is inevitable. As precision requirements become more and more strict, the error can no longer be tolerated. Although microswitches have been used to replace the light sensor 24, they cannot overcome the problems because they require very similar installation processes.